Honestly, I'm not really trying to bust on NG itself, it's a decent site and has a lot of cool stuff. But the forums...ugh. Lots of really fucking weird people. Crazy furries, proto-loveshies (I saw a lot of rants that would fit in on the loveshy forums), idiots who got 4chan memes about three years too late, people who went into way too much detail about their weird fetishes, and lots of other unpleasant internet types. If I was to hazard a guess, it's just because Newgrounds is a (relatively) mainstream site, and with all the people that frequent the forums, you get a lot of strange ones.
I posted on the forums from 06 to 08. The redesign was in 07, and there was another one after I left that I really don't care enough to look up. Last time I looked, there were a ton of crazy bronies there, which really doesn't surprise me.
I think the only reason it interests me is not because it's a particularly funny lolcow, but because I'm always like 'what the fuck was I thinking posting here?'.